Antiskid devices for vehicles are used to enhance the stability of the direction in which the vehicle moves when it is braked, to make the driver operate the vehicle with greater ease, and to reduce the braking distance. Such device consists principally of sensors for detecting the rotational frequency of each wheel, a module for analyzing the signals delivered from the sensors and delivering a signal to lessen the braking action when one or more of the wheels are nearly locked, and an actuator for decreasing the pressure of the brake fluid and restoring it to its original value according to the signal from the module.
Various arrangements are known as the aforementioned actuator. Among these configurations, a type of actuator includes cutoff valves and pistons as mentioned previously, and the pistons are moved forward by pressurized brake fluid in the passages communicating with the wheel brake cylinders and are moved backward by hydraulic pressure generated by a power unit, in order to close and open the cutoff valves and to increase and decrease the volume in each passage communicating with the wheel brake cylinders. In the actuator of this kind, variations in the brake fluid pressure are not transmitted to the hydraulic lines communicating with the brake master cylinder. Hence, the driver can operate the brake pedal with a comfortable feeling. Further, this actuator has the advantage that it is compact as compared with an actuator which utilizes the negative pressure inside the engine-intake-manifold rather than the hydraulic pressure generated by a power unit as mentioned previously.
As described in Japanese Patent Laid-Open No. 26658/1983, the pump for the power steering device of a vehicle has heretofore been used as the aforementioned power unit. In the power steering device, a mineral oil is employed as the medium for transmitting hydraulic pressure. On the other hand, the brake fluid consists of a vegetable oil. Mineral oil and vegetable oil vary considerably in physical characteristics. Thus, if the hydraulic fluid in the power steering device intrudes into the brake lines, or inversely if the brake fluid intrudes into the hydraulic lines of the power steering device, the lines will cause problems. To prevent such an undesirable situation, an atmospheric chamber has been formed between each brake fluid seal for hermetically isolating the piston from the cylinder and each seal for the hydraulic pressure generated by the power unit so that liquid leaking out of either of these two seals may escape into the atmosphere. However, the atmospheric chamber adds to the length of the combination of the piston and the cylinder. Further, installation of other seals is needed to prevent the air inside the atmospheric chamber from intruding into the hydraulic lines. This further increases the length of the combination of the piston and the cylinder and requires additional components, resulting in an increase in the cost. In addition, the maximum hydraulic pressure that can be produced by the pump for the power steering device is much lower than the maximum pressure of the brake fluid which causes the wheels to be almost locked, and therefore the pistons and the cylinders must be designed to have stepped portions. This requires a cumbersome process to machine them, contributing to an increase in cost.